In a motor control device used for an electric power steering system (EPS), or the like, a driving circuit (PWM inverter) that supplies driving electric power to a motor based on motor control signals is usually formed by connecting pairs of serially connected switching elements (switching arms), in correspondence with respective phases, in parallel with one another. Some of such motor control devices have current sensors at the low potential sides (grounding sides) of the switching arms that constitute the driving circuit. For example, see Patent Document 1.
In usage where smooth motor rotation and high quietness are required, like in an EPS, driving electric power is generally supplied to the motor by sinusoidal wave conduction; however, feedback of current values of the respective phases is indispensable for that supply. Therefore, the current sensors for current detection of the respective phases are provided at the driving circuit that is a driving electric power output unit.
In such a motor control device, current values of the respective phases are detected by the current sensors provided at the low potential side of the driving circuit at the timing at which all the low potential side switching elements that constitute the driving circuit are turned ON.
Specifically, as shown in FIG. 9, motor control signals are usually generated based on a comparison between duty command values (Du, Dv, Dw) of the respective phases, computed by executing current feedback control, and triangular waves (δ1, δ2). Note that, in this example, at the time of ON/OFF of each of the switching elements that constitute the driving circuit, in order to set a dead time for preventing a short circuit (arm short circuit) between the high potential side switching element and the low potential side switching element in each switching arm, vertically shifted two triangular waves δ1 and δ2 (δ1>δ2) are used.
That is, when the duty command value Du, Dv or Dw is higher than the value of the triangular wave δ1, a motor control signal for turning on the high potential side switching element corresponding to that phase is generated; whereas when the duty command value Du, Dv or Dw is lower than the value of the triangular wave δ2, a motor control signal for turning on the low potential side switching element corresponding to that phase is generated. Then, a current value of each phase is detected at the timing at which the triangular wave δ1 or δ2 used to generate the motor control signal reaches a “peak”.
However, although current detection is performed at the timing at which all the low potential side switching elements are turned ON as described above, current detection requires a certain period of time. Thus, at the time of an increased duty command value Du, Dv or Dw, the ON time t0 of the low potential side switching element corresponding to that phase becomes shorter than the detection time ts of a current value of that phase, so there occurs a case where the current detection cannot be performed. Therefore, in existing art, in order to ensure the detection time ts of a current value of that phase, in consideration of the detection time ts, for example, the dead time td for turning off both switching elements in order to prevent the arm short circuit is added to the detection time ts as a margin to set a high limit value Dmax for the duty command values Du, Dv and Dw of the respective phases.